1. Field of the Invention
This invention relates in general to horizontal belt filters and, in particular, to a belt filter with a movable roller deck for optimizing the alignment of the belt with the vacuum pan underlying the belt.
2. Description of the Related Art
Horizontal belt filters have been used for some time in a variety of applications that require continuous filtration of slurries, such as in processing minerals, ores, chemicals, wastes, food and wood pulp. Belt filters consist of a filter medium overlaying a conveyor belt with drain holes in communication with a vacuum pan. The slurry is fed continuously at one end of the belt and the filtrate is extracted through the drain holes as the belt transports the filtered solids to the discharge end of the belt, where they are discharged.
FIG. 1 illustrates the typical configuration of a commercial horizontal belt filter. Feed slurry (typically a water/solid-particle mixture) is uniformly distributed at the system's feed end 10 over the full width of a filter medium 12 by a top-delivery feed system 14 tailored to the particular process requirement. The filter medium 12 is supported by a belt 16 that travels continuously between the feed and the discharge ends of the system. Dewatering of the slurry is accomplished during transport by applying vacuum to the bottom of the belt 16 through a series of drainage holes positioned at the center of the belt.
As seen clearly in FIG. 2 (through the cut-out filter medium 12) and in the section of FIG. 3 (without the filter medium), such drain holes 18 are typically spaced a few centimeters apart along the longitudinal axis of the belt 16 and are aligned with a vacuum pan 20 underlying the belt. The vacuum pan 20 includes a narrow longitudinal slot 22 facing the bottom of the holes 18 in sealing contact with the underside of the belt 16. The pressure differential created at the vacuum pan by a remote vacuum pump P draws the slurry liquor through the filter medium 12, typically a cloth, along transverse channels 24 in the belt 16 and then through the drain holes 18 centered over the vacuum pan 20. The channels and drain holes are carefully sized to accommodate the hydraulic and pneumatic flow requirements for efficient filtration.
A wash system 26 may be utilized to meet specific process requirements as the cake produced by the dewatered slurry travels down the belt line. Horizontal belt filters may span as much as about 45 meters in length and about 7 meters in width; therefore, multiple washing zones may be utilized along the belt line to optimize product recovery. The filtrate and the air drawn by the vacuum acting on the drain holes 18 are collected in the vacuum pan 20 along the entire length of the drainage belt 16 and further in a collection vessel 28 where the liquid is de-aerated and pumped away from the system. The dewatered cake C is separated from the filter medium 12 at the discharge end 30 of the system as the filter medium rotates around a small discharge-end roller 32 after separating from the drainage belt 16 as the belt revolves around the drive pulley 34. Multiple wash sprays 36 are used to clean the filter medium 12 and the belt 16 on the return side of the system. Continuous tension of the belt 16 is provided by adjusting the position of the take-up pulley 38 at the feed end 10 of the system and the filter medium 12 is similarly kept in tension by a gravity take-up roller 40.
Obviously, the operation and efficiency of horizontal belt systems depend in large measure on the proper alignment of the drain holes 18 with the narrow channel defined by the slot 22 in the vacuum pan 20 underlying the belt line. As those skilled in the art know well, the tension applied to the belt 16 by the drive pulley 34 tends to cause the belt to wander laterally in response to varying friction effects encountered along the line. As soon as a drain hole 18 becomes misaligned with the slot 22 of the vacuum pan, suction from that hole is lost and the efficiency of filtration is reduced. This problem has been particularly pronounced in longer belt filters and the installation of side rollers to contain the belt within a well-defined space has not produced significant improvements. Therefore, this loss of efficiency has limited the development of higher-throughput units. The present invention is directed at solving this problem.